A power supply for providing a desired output voltage is absolutely necessary in order to cause an electronic circuit or electronic circuitries for various electronic apparatus, applied electronic apparatus or the like to operate properly. It is generally impossible or very difficult to obtain such output voltage directly from a voltage source such as a battery or the like. Particularly, a power supply for driving an electronic apparatus installed in an artificial satellite, a planet explore or the like employs a DC-DC converter for controlling the voltage of a solar panel as an input power supply to provide a desired output voltage. In such DC-DC converter, it is required to provide a stabilized output voltage by stepping-up or stepping-down a largely fluctuating input power supply with low noise and power loss. Conventional examples or general technologies of utilizing a solar panel as an input power supply will be described hereunder.
In case of a stepping-down DC-DC converter, it is necessary to adjust the number of solar panels to be connected in series so that the output voltage from the solar panels (i.e., the input power source voltage) is always higher than the output voltage of the DC-DC converter. If the voltage from the solar panels fluctuates over a wide range, the maximum or peak output voltage from the solar panels tends to be very high, it is difficult to properly design the DC-DC converter.
On the other hand, in case of using a stepping-up DC-DC converter, the number of solar panels to be connected in series must be adjusted so that the output voltage from the solar panels is always lower than the output voltage from the DC-DC converter. If the voltage from the solar panels fluctuates largely, it is also difficult to properly design the DC-DC converter in this case because the output voltage from the solar panels could become very low.
Accordingly, it is possible to properly set the range of the output voltage from the solar panels when a stepping-up/down DC-DC converter in which the input voltage from the solar panels can be stepped-up or down to the output voltage of the DC-DC converter, i.e., the voltage that is required to supply to a load. However, a non-isolated stepping-up/down DC-DC converter with low power consumption has such a problem that the input and output voltages are in opposite polarity to each other and thus difficult to handle. Moreover, it is essential that the DC-DC converter to be used for the aforementioned planet explorer or the like causes minimum noise (switching noise) because its primary purpose is to observe very weak electric field or magnetic field in the vicinity of planets.
Now, a brief description will be given on a typical non-isolated stepping-up/down switching DC-DC converter with reference to FIGS. 12-16. All of these DC-DC converters have common problems that the input and output voltages are opposite polarity to each other and the input and output currents are pulse waves, thereby exhibiting large noise. This means that the pulsating input and output currents have large amplitudes at the switching frequency and a large rate of change in time, thereby providing a large noise at the switching frequency as well as large harmonic noises at the frequencies equal to the switching frequency multiplied by any integer.
FIG. 12 is a first example of conventional DC-DC converters (or a Buck-Boost converter), wherein (A) is a circuit schematic, (B) is a transfer function, (C) is a ripple current, (D) is a ripple voltage and (E) is the voltage across the coil L in FIG. 12(A). As shown in FIG. 12(A), the DC-DC converter 12 comprises an input voltage source E, a switch S, a coil (or inductor) L, a diode D, a load resistor Ro and a load capacitor Co. The switch S and the coil L are connected in series across the input voltage source E. Also, the load resistor Ro and the load capacitor Co are connected in parallel across the both ends of the coil L by way of the diode D. It is assumed that the voltage of the input voltage source E and the output voltage across the load resistor Ro are Vi and Vo, respectively.
When the switch S is periodically turned ON and OFF in the DC-DC converter 12 as shown in FIG. 12, the ripple current as shown in FIG. 12(C) flows through the switch S and the coil L and develops a coil voltage of a square pulse that varies between −Vo and Vi as shown in FIG. 12(E), thereby supplying an output voltage −Vo to the load Ro. In other words, the input voltage Vi and the output voltage Vo of the DC-DC converter 12 are opposite to each other and the input current Ii and the output current Io are pulse waves.
Now, FIG. 13 is a second example of conventional DV-DC converter (or a Cuk converter), wherein (A) is a circuit schematic, (B) is a transfer function, (C) is a ripple current, (D) is a ripple voltage and (E) is a coil voltage. As shown in FIG. 13, the DC-DC converter 13 comprises an input voltage source E, a first coil L1, a switch S, a capacitor C1, a diode D, a second coil L2, a load resistor Ro and a load capacitor Co. The first coil L1 and the switch S are connected in series between both ends of the input voltage source E. The capacitor C1 and the diode D are connected in series across the switch S. Moreover, the load resistor Ro and the road capacitor Co are connected in parallel across the diode D by way of the second coil L2.
In the DC-DC converter 13, ripple currents through the first coil L1 and the second coil L2 are shown in FIG. 13(C). The ripple currents shown at the top are the case when there is no magnetic coupling between these coils L1, L2. The ripple currents shown in the middle are the case when the magnetic coupling coefficient k between these coils L1, L2 is equal to n. On the other hand, the ripple currents at the bottom show the case when the magnetic coupling coefficient k between these coils L1, L2 is equal to 1/n. Although the input and output ripple currents can be made to combination of a triangle wave and a 0 ripple or a 0 ripple and a triangle wave, it is to be noted that the input voltage Vi and the output voltage Vo are opposite polarity to each other similar to the case in the aforementioned DC-DC converter 12.
Now, shown in FIG. 14 is a third example of conventional DC-DC converters (or a Cuk converter with an intermediate coil), wherein (A) is a circuit schematic, (B) is a transfer function, (C) is a ripple current, (D) is a ripple voltage and (E) a voltage across the coil. As shown in FIG. 14(A), the DC-DC converter 14 comprises an input voltage source E, an input coil L1, a switch S, a pair of capacitors C1, C2, an intermediate coil Lm, a diode D, an output coil L2, a load resistor Ro and a load capacitor Co. The input coil L1 and the switch S are connected in series between both terminals of the input voltage source E. The capacitor C1 and the intermediate coil Lm are connected in series between both terminals of the switch S. The capacitor C2 and the diode D are connected in series between both terminals of the intermediate coil Lm. Moreover, the load resistor Ro and the load capacitor Co are connected in parallel between the both ends of the diode D by way of the output coil L2. It is to be noted that the input coil L1, the intermediate coil Lm and the output coil L2 can be magnetically coupled in a predetermined polarity relationship.
Shown in FIG. 14(C) are generally triangle ripple currents through the input coil L1, the intermediate coil Lm and the output coil L2 sequentially disposed in left to right directions. Ripple voltages developed by these ripple currents across such coils are also shown in FIG. 14(D). As shown in FIG. 14(C), both of the input current through the input coil L1 and the output current through the output coil L2 in the DC-DC converter 14 can be made substantially 0 ripple. However, similarly to the aforementioned DC-DC converters 12 and 13, the input voltage Vi and the output voltage supplied to the load are opposite polarity to each other.
Now, shown in FIG. 15 is a fourth example of conventional DC-DC converters (or a Zeta converter), wherein (A) is a circuit schematic, (B) is a transfer function, (C) is ripple currents, (D) is ripple voltages and (E) is a coil voltage. As shown in FIG. 15(A), the DC-DC converter 15 comprises an input voltage source E, a switch S, an input coil L1, a capacitor C1, a diode D, an output coil L2, a load resistor Ro and a load capacitor Co. The switch S and the input coil L1 are connected in series between both ends of the input voltage source E. The capacitor C1 and the diode D are connected in series between the both ends of the input coil L1. The load resistor Ro and the load capacitor Co are connected in parallel between the both ends of the diode D by way of the output coil L2. The input coil L1 and the output coil L2 are magnetically coupled to each other.
As shown in FIG. 15(C), the currents flowing through the switch S, the input coil L1 and the output coils L2 change depending upon the magnetic coupling factor k between the both coils L1 and L2. Shown at the top are the currents when there is no magnetic coupling between the both coils L1 and L2. Shown at the middle are currents when the magnetic coupling factor k is equal to n. Shown at the bottom are currents when the magnetic coupling factor k is equal to 1/n. Although the input voltage Vi and the output voltage Vo are the same polarity in the DC-DC converter 15, the input current Ii is pulsating.
Now, FIG. 16 shows a fifth example of conventional DC-DC converters (or a Sepic converter), wherein (A) is a circuit schematic, (B) is a transfer function, (C) is ripple currents, (D) is a ripple voltage and (E) is a coil voltage. As shown in FIG. 16(A), the DC-DC converter 16 comprises an input voltage source E, an input coil L1, a switch S, a capacitor C1, an output coil L2, a diode D, a load resistor Ro and a load capacitor Co. The input coil L1 and the switch S are connected in series between both terminals of the input voltage source E. The capacitor C1 and the output coil L2 are connected in series between the both ends of the switch S. Moreover, the load resistor Ro and the load capacitor Co are connected in parallel between the both ends of the output coil L2 by way of the diode D. The input coil L1 and the output coil L2 are magnetically coupled to each other with a magnetic coupling factor k.
As shown in FIG. 16(C), the ripple currents flowing through the input coil L1, the output coil L2 and the diode D change in triangle pulse waves depending upon the magnetic coupling factor k between the input coil L1 and the output coil L2. Although the input voltage Vi and the output voltage Vo to be supplied to the load are the same polarity to each other in the DC-DC converter 16, the output current Io is pulsating.
As described hereinabove, the conventional DC-DC converters as shown in FIGS. 12-16 are difficult to handle because of opposite polarity between the input and output voltages or difficult to be applied to such applications critical to noise because they are subjected to a large noise due to ripples in the input and output currents.